Advice & Reviews
June 2, 2016

No In-Between: Insulating a Pitched Roof

As we all learnt when a bobble hat was forced on our head on the way out to play, heat rises. Therefore, it’s common sense that a well-insulated roof is a smart investment for a homeowner looking to improve the energy efficiency of their home, be that newly built or as part of a refurbishment.

The best way to insulate a pitched roof is by installing insulation between and above the rafters. This approach can be used to create an unventilated roof structure, helping to keep out unwanted drafts. As there’s no insulation below rafters, headroom is maximised, creating more useable space. Of course, stripping the entire roof isn’t practical in many refurb applications so an alternative method is to insulate between and under the rafters and to ventilate the space.

We offer build-ups for both of these approaches on our U-value Calculator, but there’s no option for just installing insulation between the rafters. Why not? Surely that would save money, time and space?

The answer has a lot to do with the Building Regulations/Standards.  From Part L of the Building Regulations (England and Wales) or Section 6 in Scotland, we recommend that the U-value of a pitched roof, insulated at rafter level should be 0.18 W/m2.K for a domestic refurbishment. For a new build property we recommend a target U-value of 0.11 W/m2.K for properties in England and Wales and 0.10 W/m2.K for Scottish homes.

If you are constructing a ventilated roof (insulating between and under the rafters), then there needs to be a 50 mm air gap between the insulation and the sarking felt to avoid condensation building up. For an unventilated roof without the use of counter battens, a gap would need to be left for the breathable membrane to drape. Both of these requirements will invade the space available for the insulation between the rafters.

Then there is the issue of rafter depth. Common rafter depths range from 100 mm to 200 mm. This means that, all measurements considered, the maximum thickness of the insulation would be limited to a thickness of between 50 and 175 mm. This is unlikely to be enough to achieve the U-Values needed to meet Building Regulations/Standards and therefore more insulation would be needed.

Adding another layer above or beneath the rafters also offers another key advantage. Obviously, heat will be lost faster through the rafters than the insulation, so an added layer of insulation reduces the effect of this thermal bridging through the timber, helping to keep the space nice and cosy.

And it’s really as simple as that. Want to get the best results and stay the right side of the Building Regulations or Standards? Go with insulation between and over or between and under the rafters.

Getting to Know… Kingspan Kooltherm K7 Pitched Roof Board

Kingspan Kooltherm K7 Pitched Roof Board is a premium performance rafter-level insulation for ventilated or unventilated pitched roofs. Suitable for new build and refurbishment projects, it can be installed in between and above or between and below rafter applications.

Whether the loft space is for the Christmas decorations, or it has been transformed into an extra bedroom, keeping internal build-ups as slim as possible is essential to maximise the amount of usable space. Thanks to its premium performance core, Kingspan Kooltherm K7 Pitched Roof Board can achieve thermal conductivities as low as 0.020 W/m.K. As a result, you can achieve compliance with a slimmer thickness of insulation either above or below the rafters.

Its rigid and lightweight design makes it easy to install and cut with a sharp knife (and get up the ladder in the first place!).

Don’t forget, if you need assistance calculating what type and thickness of insulation you need for an application, simply visit Kingspan Insulation’s free online U-value Calculator. The calculator is also available as a free app for Windows, Apple and Android devices.